Playing God or Playing Man?
One could suggest the following explanation:
Even though Yosef may have forgotten his dreams for some twenty years, when his brothers arrive in Egypt and bow down to him, it suddenly dawns on him that his childhood dreams may actually be coming true after all. If Binyamin is still part of the family, as his brothers claim, then the "bechira" process may have actually included Bnei Rachel. (See Board #3.)
Suddenly, all the events that transpired become pieces of a much larger puzzle. Yosef suddenly realizes as well that the family must ultimately unite, with himself responsible for its leadership (as his dreams suggested). Yosef now sees God's fingerprints on every piece of the puzzle, but he cannot reveal himself to his brothers for a very simple reason. If he did so immediately, a wound would be opened that could never be healed. Let's explain:
Yosef does not need to play God, to ensure that his brothers do teshuva; that is God's responsibility. Yosef, however, does have to play leader. Given the situation created by "mechirat Yosef," had Yosef revealed his identity at this point, his brothers would have 'melted' on the spot! How could they face him? How could they ever be able to speak to him? The shame of their relationship with him could create an eternal barrier between them.
Although they all may have remained 'chosen,' family unity and fraternity would have been impossible to achieve. Therefore, Yosef finds it necessary to do something that would reunite the family. He needs to create a situation that will bond the family both physically and spiritually. [Indeed, this is the mark of a true leader.]
Furthermore, had Yosef informed his father at this point (before informing his brothers), then Yaakov's anger toward his sons would have undoubtedly brought him to curse them. This, for sure, would yield disastrous results for the future of Am Yisrael. Yosef must restrain his emotions for a few more weeks in order to create a situation of true family unity.
With this background, once can suggest that Yosef's primary goal from the very beginning is to create a situation whereby the brothers can redeem themselves, by putting them through a difficult test: the brothers must forfeit their lives in order to save Binyamin. [Yosef also does many other things to make the brothers wonder and think, to shake them up a bit - "cheshbon ha'nefesh".] By planting his cup in Binyamin's bag, not only will Yosef find out whether the brothers had done teshuva, but the brothers now have an opportunity to prove to themselves that they have done teshuva - that they are indeed men of virtue! Only after they demonstrate willingness to give up their lives for Binyamin will they be able to face Yosef as a brother, and unite once again as a complete family.
Finally Yosef will reveal himself (in Parshat Vayigash), but despite his intense desire to do so, he holds himself back until he has a created a situation where the brothers cannot only look at him, but will also be able to 'look at themselves.'